1. Field of the Invention
The present invention relates to a fixing belt which is utilized in an electro-photographic apparatus and the like and to a method for evaluating the fixing belt.
2. Description of the Prior Art
In general, there is an electro-photographic apparatus and the like in which a belt fixing device having a fixing belt utilized is equipped.
As one example, such a belt-fixing device is disclosed in Japanese Patent Laid-Open Publication No. Hei 06-318001. The belt-fixing device disclosed in the publication is constituted so that it includes a fixing roller and a heating roller, and an endless like fixing belt having on a surface thereof a mold releasing layer is loaded between the both rollers, a pressure roller is disposed under the fixing roller to pinch the fixing belt to pass a paper through a nipping portion between the fixing belt and the nip roller.
Further, the belt-fixing device is provided with a preheating process which is arranged between the heating roller and the nipping portion to preheat toner beforehand.
However in the prior art belt fixing device, because the fixing roller is made of solid rubber, it is not possible to absorb a stress caused by thickness of the paper, a pressure distribution in the nipping portion is in an uneven state such that the pressure is low around an entrance and an exit of the nipping portion and is most high at a center part of the nipping portion. On the other hand, because water included in the paper is evaporated with heat brought by the fixing belt while the paper is brought through the preheating process, there is a high possibility that partial convexes and concaves are generated on the paper. When the paper having such convexes and concaves passes through the nipping portion having such uneven pressure distribution, a portion where the paper is partially restrained strongly is generated to further exaggerate the convexes and concaves thereby occurring crumples on the paper.
To prevent an occurrence of such crumples on the paper, for example, in Japanese Patent Laid-Open Publication No. Hei 10-274894 there is disclosed a method that a circumferential speed ratio between the fixing roller and the nip roller is set in a range of 0.97-1.03 and a delay of the nip roller is inhibited in order to reduce a stress suffering on the paper and thereby the occurrence of crumples on the paper are prevented.
Also, for example, in Japanese Patent Laid-Open Publication No. Hei 7-92840 there is disclosed a method that a covering layer of fluorine containing resin is arranged on an elastic layer on a conductive axis body of the fixing roller and the nip roller to prevent an occurrence of the crumples on the material paper.
Further, in Japanese Patent Laid-Open Publication No. 2001-282033 there is disclosed a method that temperature around edge sides of a paper passing part in the nipping portion along axial direction of the fixing roller and the nip roller are set higher than that of central part side to prevent an occurrence of the crumples on paper.
It is important that a surface of the fixing belt has an appropriate flexibility in order to prevent the occurrence of crumples of this kind.
However, any one of the above described prior arts is not based on flexibility of the surface of fixing belt, and there has no method for evaluating flexibility of surface of the fixing belt, so that only a visual examination and so on has been carried out with respect to the fixing belts delivered from the manufacturer of parts.
In this way, because there is no method for evaluating about whether or not the fixing belt has appropriate flexibility at its surface in the prior art, all the fixing belts which pass the visual examination and so on, are adapted to be utilized on the belt-fixing device. As a result, when in a case a fixing belt that has not appropriate flexibility at its surface, comes to be mixed in the delivered lot, crumple is generated on the paper and thereby it causes degradation in quality level though several kinds of countermeasures are introduced and achieved.